Apparatus for stirring liquid materials, including culture mediums, have long been known. For example, Scharf et al. U.S. Pat. No. 3,649,465 provides a flask or vessel having an opening at its upper portion, with a closure for the opening, having a spindle extending therethrough, the spindle at its lower end having a magnetic stirrer, with a shroud extending in surrounding relationship to the spindle. The stirrer is driven magnetically by a driving magnet, and the magnetic stirrer, located at the bottom of the flask, may be adjusted through a limited vertical range by vertically adjusting the spindle within the limits permittted by the shroud.
Harker et al. U.S. Pat. No. 2,958,517 provides a flask having a rod guided in a bearing in a closure for the flask, the rod having at its lower end a magnetic impeller, which engages the bottom of the flask, the magnetic impeller being driven by a magnetic stirring apparatus on which the flask is held, the apparatus including an electric motor having a shaft driving a magnet which is magnetically coupled to the magnet within the flask.
Harker U.S. Pat. No. 3,572,651 provides a flask having a closure provided with a bearing on its underside, the bearing supporting a spindle having at its lower end, near the bottom of the flask, a magnetic stirrer, the magnetic stirrer being driven by a conventional magnetic driving apparatus.
Mazowski U.S. Pat. No. 3,622,129 also discloses a magnetic stirrer apparatus, in which a flask has an opening, a closure for the opening with a rod extending through the opening, the rod supporting at its lower end a magnetic stirrer, and the rod being adjustable, vertically, in the closure, so as to position the stirrer at different depths in the liquid in the flask.
Sada et al. U.S. Pat. No. 4,310,253 discloses an apparatus in which a vessel containing a body of liquid has floating, magnetic particles which are caused to rotate by a rotating magnetic field, to rotate the interface between, for example, a liquid and a gaseous body in the vessel.
The prior art in which the stirrer is submerged in the liquid was subject to various defects and deficiencies, including constructions which were difficult to clean and which did not have sufficient cell proliferating action. In some cases, obstructions were provided to the liquid motion by the stirrers, or stirring action unsuitably vigorous for cell culture was required to insure complete stirring action.